The invention lies in the field of recovering aluminum from its ores.
The present invention is an improvement in the basic process disclosed in the above-referenced and incorporated patents for recovering aluminum from alunite ore, the basic process broadly comprising dehydrating the particulate ore by roasting, removing sulfur and alkali metal compounds from the roasted ore by one of the alternatives of a reducing roast followed by an oxidizing roast and leaching with solvent, as disclosed in U.S. Pat. No. 3,890,425, or by a direct alkaline leach, as disclosed in U.S. Pat. No. 3,890,426, resulting in a particulate residue containing aluminum values and a solution containing potassium sulfate, which may optionally be recovered, digesting the residue with alkaline solution to convert the aluminum values to soluble aluminate, removing silicon from the aluminate solution, and precipitating the aluminum values as aluminum hydroxide from which alumina is recovered. The expression "roasting and leaching to remove water and compounds of sulfur and alkali metals" as used herein includes the above alternatives.
The invention is an improvement in the above process in which the particulate residue from the leaching step is digested in the Bayer leach in multiple stages by separating it into two fractions, digesting one fraction first with at least one alkali metal hydroxide to a point below saturation of the digestion liquor to convert aluminum values in the fraction to soluble aluminates, and then digesting the second fraction with the liquor resulting from the first digestion step. Preferably, the fraction having the highest extractable aluminum content is digested first. In the preferred embodiment of the invention, one fraction is made up of relatively large particles and the other fraction is comprised of relatively small particles. In this situation, the smaller particle size fraction usually has the higher aluminum content and would ordinarily be digested first although the larger particle size fraction may be digested first.
The processes for recovering aluminum from its ore are highly competitive, and it is important that a competitive process include refinements leading to the highest possible recovery of aluminum. This is particularly true of processes for the recovery of aluminum from low grade ores like alunite, which processes are in competition with those for the recovery of aluminum from bauxite.
While the basic process referred to is highly effective for recovering aluminum from alunite ores, it is, or course, subject to improvement. In the digestion step of the basic process, it is advantageous for maximum percentage recovery of aluminum that the liquor of soluble aluminates resulting from the digestion procedure have a high aluminum to caustic (A/C) ratio. If the solids content of the digestion liquor is increased during digestion of the particulate residue to the point to provide enough aluminate to give a high (A/C) ratio in a single digestion step, a maximum extraction of aluminum is not achieved because aluminum extraction efficiency decreases as the solids content in the alkali metal digestion liquor increases beyond a certain point. Aluminum extraction is apparently improved by lowering the percent solids up to the point of maximum aluminum extraction efficiency.
Accordingly, it is an object of this invention to provide an improvement in the Bayer digestion step of the basic process resulting in the liquor from the digestion step having a high A/C ratio and low solids content, and resulting in a higher percentage recovery of aluminum during the subsequent precipitation step of precipitating aluminum hydroxide from the digestion liquor.